1) Field of the Invention
The present invention relates to a method of and an apparatus for inspecting defects such as flaws, projections, dust, etc. on a surface of a test piece, particularly on a coated surface of a magnetic disc.
2) Description of the Related Art
Heretofore, an inspection of a coated surface of a magnetic disc employed in a recording apparatus of a large-type computer has visually been made by an operator, for example. However, it is hard to uniformly perform its inspection due to the fact that there is a personal difference in a defect-determination criteria among operators. In recent years, it has also been difficult to visually inspect defects because the minimum dimension of a detected defect is on the order of a micron with the advancement of rendering the recording density of the magnetic disc high.
When it is desired to inspect defects such as flaws, projections, etc. present on the coated surface of the magnetic disc, for example, there is known a method in which the surface of the magnetic disc is photographically recorded by a television camera or the like so as to detect image signals, and the thus-detected image signals are represented in binary form based on threshold values, thereby recognizing defects such as the flaws, the projections on the basis of the image signals represented in binary form. When the surface to be inspected is in a stable state, such a method permits the extraction of defects by representing a detected signal from a normal surface in binary form based on a predetermined threshold value because the detected signal is constant in level.
However, when the thickness of coatings or films on the surface of the magnetic disc varies and the surface to be inspected is in an unstable state, a detected signal varies as shown in FIG. 13. When it is desired to discriminate among fine defects (a) to (b) by fixed threshold values, even a variation in a background level would be represented in binary form, thus making incapable of recognizing defects. As a method of overcoming such a drawback, there is known one of representing detected signals in binary form of a floating type as disclosed in Japanese Patent Publication Laid-Open No. 54(1979)-3638, for example.
This method is used to reduce scaled-down and smoothed signals L.sub.1, L.sub.2 in level from a detected signal S so as to add a predetermined level to the thus-reduced signals and subtract the same from the signals, thereby representing the results of its addition and subtraction as threshold values in binary form. According to this method, it is effective in inspecting defects on a printed-circuit board, for example. However, the method has a problem that although the defects (a), (b) and (d) can be discriminated as shown in FIG. 14 on a coated surface of a magnetic disc or the like, the defects (c) and (e) cannot be detected.
On the other hand, threshold values L.sub.1', L.sub.2' should be set as shown in FIG. 15 in order to detect even the defects (c) and (e). However, even the representation of a background level in binary form is performed as well as representation of the defects (c) and (e) in binary form in this case, thus causing a problem that false detection is performed.
In addition, a floating-threshold method is accompanied by the problem that since it makes use of dynamic response of an electric circuit, it can hardly be applied to variations in application of coatings caused in a manufacturing process as in a coated surface of a magnetic disc.
Thus, a detected signal indicative of a defect present on the coated surface of the magnetic disc is represented as shown in FIG. 12. It is hard to discriminate between the level of the detected signal and a noise level, thereby causing a problem that one fails to detect defects.
In order to solve these problems, there are known methods disclosed in Japanese Patent Publication Laid-Open Nos. 61(1986)-256242 and 62(1987)-223652.
The former discloses a method wherein a test piece such as a film of coating on a magnetic disc is scanned and recorded by photography so as to be associated with an image pick-up device, and gate signals having widths in level equivalent to dimensions of defects such as flaws, projections on a surface of the test piece are produced to be overlapped to each other, thereby making determination of the presence of a defect when the difference between the maximum and minimum values of image signals obtained from the image pick-up device within a period in which the gate signals are present exceeds a prescribed threshold value. However, the former has the problem that when a fine defect appears, it is necessary to narrow the width in level of each gate signal, thus causing difficulty in discriminating between noise components and defect components.
The latter describes a method comprising the steps of scanning and recording by photography a test piece such as a film of coating on a magnetic disc so as to be associated with an image pick-up device; producing gate signals having widths in level equivalent to dimensions of defects such as flaws, projections on a surface of the test piece to be overlapped to each other; subtracting a signal of low-frequency components out of an image signal obtained from the image pick-up device from the image signal obtained from the image pick-up device so as to determine a difference signal therebetween; integrating the difference signal over a time-width of each gate signal; and making a judgment of the presence of a defect when the value thus integrated converted into the absolute value exceeds a prescribed value.
The latter also has the problem that although a large defect can be detected in the same manner as the former, it is necessary to narrow the width in level of each gate signal in a case of presence of a fine defect, thus causing difficulty in discriminating between noise components and defect components.
As prior arts, there are known U.S. Pat. Nos. 4,674,875; 4,423,331 and 4,395,122.